1. Field of the Invention
The present invention relates to an inkjet printer, an inkjet printer controller and a method for controlling the inkjet printer.
2. Discussion of the Background
An inkjet printer is an apparatus for printing information such as characters, figures, patterns on a printing surface of a print medium by ejecting microparticles of ink from a printer head while moving the printer head in the anteroposterior direction and the lateral direction relative to the print medium. The printer head has a plurality of nozzles for ejecting ink which are aligned in the anteroposterior direction at a predetermined pitch. While the ink ejection from the respective nozzles is controlled by an ejection controller, ink is ejected from the nozzles with moving a carriage in a lateral direction by a lateral moving mechanism so as to form a band-like printing area of which width, i.e. print width, corresponds to the width in the anteroposterior direction of the nozzle array from a nozzle at the front end to a nozzle at the rear end of the printer head and which extends in the lateral direction. This action is carried out in combination with the relative movement in the anteroposterior direction of the print medium by the anteroposterior moving mechanism, thereby printing desired information on the printing surface.
Recently, there is a market demand for high speed printing, so a multi-head inkjet printer having a plurality of printer heads as mentioned above which are aligned in the anteroposterior direction in a carriage become popular. For example, in a multi-head inkjet printer having four printer heads which are shifted from each other in the anteroposterior direction and the lateral direction so that they are arranged in a zigzag shape, so-called “staggered arrangement”, respective nozzle arrays of the four printer heads are configured to be continued in the anteroposterior direction as seen from the side of the carriage so that the printer can print at once four times as wide as a print width of a single printer head from a nozzle at the front end of a printer head held at the front side of the carriage to a nozzle at the rear end of a printer head held at the rear side of the carriage. For example, JP-A-2000-62153 discloses such a printer. The contents of this publication are incorporated herein by reference in their entirety.
Since each of printer heads used in a multi-head inkjet printer shares a band range including printing information which is divided in the anteroposterior direction, printer heads having the same nozzle diameter and array are employed for the multi-head inkjet printer. Further, when printing on a print medium with uniform color density (brightness of color and color saturation), an ejection controller for controlling the ink ejection of the respective printer heads outputs basically the same control signals as ejection control values to the respective heads. However, the ejection characteristics of the respective printer heads are actually slightly different from each other within a predetermined range even after setting predetermined parameters for each head unit so that the uniform color density over the entire print width may not be obtained even when the plurality of printer heads are controlled with ejection signal values for the uniform color density. For example, in the aforementioned multi-head inkjet printer in which four printer heads are arranged in the staggered arrangement, if the color density of a printing area printed by any one of the printer heads is lower (higher) than the color density of a printing area printed by the other printer heads, there will be a phenomenon, called “banding”, that contrasting density is striped according to the width of nozzle array of the aforementioned printer head in the entire print width of the four printer heads.
The banding is not preferable because it affects the print quality. If this phenomenon is observed, it is necessary to replace each printer head held by the carriage and then start over the adjustment setting. Accordingly, there are problems of leading to more complexity of production process and taking longer time for the production. There is a possibility that one or more of printer heads is broken or damaged and is thus required to be replaced after delivering an inkjet printer. As the aforementioned phenomenon occurs under the circumstances, there is a problem of decreasing the operation rate of the inkjet printer.